3 months were proven to be too short some years ago, after that issue,
we went far up to 12 months.
however, 12 months were considered too long after recent discussions :)
so, 6 months seems to be a good compromise.
the warning is still repeated every months and the text is unchanged.
advantage is still that this approach does not require network or
opt-in, and catches really all lazy updaters with few effort, cmp
https://github.com/deltachat/deltachat-desktop/issues/5422
